You can easily spot custom checks because they will have a few blank fields in the columns, such as "STIG Severity" will be blank. Also, if you right-click > custom check > and the "edit custom check" option is present, it is custom. Here is also where you will find what that custom check is doing via raw script.

Actually, false information on my part. the built-in RS-XX checks will NOT have a STIG severity filed present, so not a good indicator. It seems that the only quick way to spot these custom checks, is as I stated: right-click > custom check > and if the "edit/delete custom check" option is present, it is custom. This is also where you will find what that custom check is doing via raw script.